4 research outputs found
Dissecting the Red Sequence--I. Star Formation Histories of Quiescent Galaxies: The Color-Magnitude vs. the Color-Sigma Relation
We use a sample of ~16,000 non-emission line galaxies from the SDSS to
investigate the physical parameters underlying the well-known color-magnitude
and color-sigma relations. Galaxies are sorted in terms of velocity dispersions
(sigma), luminosity (L), and color, and their spectra are stacked to obtain
very high S/N mean spectra for stellar population analysis. This allows us to
map mean luminosity-weighted ages, [Fe/H], [Mg/H], and [Mg/Fe] in sigma-L-color
space. Our first result is that there are many different red sequences, with
age, [Fe/H], [Mg/H], and [Mg/Fe] showing different amounts of slope and scatter
when plotted versus sigma, L, or color. These behaviors are explained if the
star formation histories of the galaxies populate a two-dimensional parameter
space. One parameter is the previously well-known increase in age, [Fe/H],
[Mg/H], and [Mg/Fe] with sigma. In addition to this, we find systematic
variations at fixed sigma, such that more luminous galaxies are younger, more
Fe-rich, but have lower [Mg/Fe] than their fainter counterparts. The main sigma
trends support a paradigm in which more massive galaxies form their stars more
rapidly and at earlier times than less massive galaxies. The trends at fixed
sigma are consistent with scatter in the duration of star formation for
galaxies at a given sigma. The co-variation of stellar population properties
and L residuals at fixed sigma that we present here has a number of
implications: it explains the differing behavior of stellar population
indicators when investigated versus sigma as compared to L, and it reveals that
L is not as efficient as sigma for indicating galaxy "size" in stellar
population studies.Comment: 23 pages, 15 figures, Accepted to ApJ. Full resolution version
available at http://www.ucolick.org/~graves/Graves_ColorMag_fullres.ps.g
Evaluation of the vulnerability to contamination of drinking water systems for rural regions in Quebec, Canada
The aim of this paper is to describe a method for evaluating the vulnerability of drinking water systems to contamination, in particular in rural regions used intensively for agriculture. To do so, various indicators were developed to represent the source to tap multi-barrier approach for drinking water safety. These indicators correspond to four barriers: source susceptibility to contamination; water treatment efficiency; distribution system management; and, overall management of water quality. The indicators were classified, regrouped and weighted within a model based on a multi-criteria analysis. The method was developed and applied to 39 municipal water systems of rural Quebec, Canada. The model obtained can be used for planning purposes to prioritise water systems requiring improvements.drinking water, rural water, contamination, multi-criteria analysis, vulnerability,
Coronal Heating as Determined by the Solar Flare Frequency Distribution Obtained by Aggregating Case Studies
Flare frequency distributions represent a key approach to addressing one of
the largest problems in solar and stellar physics: determining the mechanism
that counter-intuitively heats coronae to temperatures that are orders of
magnitude hotter than the corresponding photospheres. It is widely accepted
that the magnetic field is responsible for the heating, but there are two
competing mechanisms that could explain it: nanoflares or Alfv\'en waves. To
date, neither can be directly observed. Nanoflares are, by definition,
extremely small, but their aggregate energy release could represent a
substantial heating mechanism, presuming they are sufficiently abundant. One
way to test this presumption is via the flare frequency distribution, which
describes how often flares of various energies occur. If the slope of the power
law fitting the flare frequency distribution is above a critical threshold,
as established in prior literature, then there should be a
sufficient abundance of nanoflares to explain coronal heating. We performed
600 case studies of solar flares, made possible by an unprecedented number
of data analysts via three semesters of an undergraduate physics laboratory
course. This allowed us to include two crucial, but nontrivial, analysis
methods: pre-flare baseline subtraction and computation of the flare energy,
which requires determining flare start and stop times. We aggregated the
results of these analyses into a statistical study to determine that . This is below the critical threshold, suggesting that Alfv\'en
waves are an important driver of coronal heating.Comment: 1,002 authors, 14 pages, 4 figures, 3 tables, published by The
Astrophysical Journal on 2023-05-09, volume 948, page 7